Motor vehicles, and in particular trucks, are a critical component of the system for transporting materials, goods and people from place to place. The amount of energy required to move such vehicles depends on many factors. For instance, a substantial amount of energy is expended to overcome the resistance encountered in moving the vehicle through air. The amount of energy expended depends in large part on the aerodynamic drag force exerted on the vehicle by the air. A vehicle moving through air experiences a drag force, which may be divided into two components: frictional drag and pressure drag. Frictional drag comes from friction generated generally through the boundary layer as the vehicle passes through the air. Pressure drag results from the net pressure forces exerted as the air flows around the vehicle. A substantial component of the pressure drag is associated with the formation of a low pressure zone behind the vehicle, as evidenced by the formation of a wake behind the vehicle.
The distinction between frictional drag and pressure drag is useful because the two types of drag are due to different flow phenomena. Frictional drag is typically most important for attached flows that is, where the flow boundary layer has not separated from the vehicle surfaces, and is related to the surface area exposed to the flow. Pressure drag dominates for separated flows, and is generally related to the cross-sectional area of the vehicle facing the air flow. When the drag on the vehicle is dominated by pressure drag forces, it will expend far more energy traveling through air than the same vehicle dominated by friction drag forces. It is therefore advantageous in the design of a vehicle to reduce pressure drag forces, thereby increasing the aerodynamic properties and efficiency of the vehicle.
A bluff body, such as a conventional truck hood or front section, produces significant pressure drag at typical highway speeds. One reason for the large pressure drag is the presence of a sharp angle located at a leading edge of the truck hood. More specifically, typical truck front sections include a substantially vertical front surface or grille that meets, along an upper edge, a substantially horizontal top surface. The air flow passing over the front section, therefore, must negotiate an abrupt change in direction as the edge where the hood structure transitions from a substantially vertical orientation to a substantially horizontal orientation. This abrupt turn causes the flow to ‘separate’ from the top surface of the hood, forming a highly turbulent region of air located directly above the top surface of the hood, between the leading edge and the windshield.
A tractor-trailer combination is another vehicle that experiences excessive aerodynamic drag. Generally described, tractor-trailer combinations typically include a tractor having a so-called fifth wheel by which a box-like semi-trailer may be attached to the tractor by an articulated connection for transportation of the semi-trailer. By providing the articulated connection, a space or gap is formed between the rear wall of the tractor cab and the forward wall of the semi-trailer. It is well known that this gap, or the gap between succeeding trailers of a tractor trailer combination, causes wake regions and, as a result, pressure drag.
Pressure drag on tractor-trailer combinations is not limited to the frontal area of the tractor and/or the gap disposed between the tractor and the articulating trailer. It is well known that the rear end of bluff bodies, such as trailers, are known to contribute significantly to aerodynamic drag, as evidenced by the formation of a wake in the trailing region behind the trailer. The generation of the wake, formed by eddies, can be contributed to the shape of the conventional trailer, which is essentially a rectangular box having a flat, rectangular roof and matching floor, along with flat, rectangular side panels. The front and rear surfaces of such trailers are also generally flat rectangular surfaces. As such, current bluff bodies, such as trailers suitable for use with tractors of the Class 8 type, suffer from a low pressure zone at the rear of the trailer such that the airstream suffers from early separation, resulting in a broad eddying wake forming downstream of the separation. The net result is the creation of considerable aerodynamic drag.